1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member, a process cartridge containing the electrophotographic photosensitive member, and an electrophotographic apparatus containing the process cartridge. In more detail, this invention relates to an electrophotographic photosensitive member comprising a specific charge generation layer, a charge transport layer having a specific thickness, a support portion having a specific surface roughness, and possessing a specific electrostatic capacity. This invention also relates to a process cartridge containing the electrophotographic photosensitive member, as well as an electrophotographic apparatus containing the process cartridge.
2. Description of the Related Art
As an electrophotographic photosensitive member for use in an electrophotographic apparatus, there has been commonly used an organic electrophotographic photosensitive member formed by an organic photoconductive material serving as a charge generation substance. This is because the use of an organic electrophotographic photosensitive member can ensure a low production cost, as well as a high freedom in designing an electrophotographic photosensitive member. Another advantage of using an organic electrophotographic photosensitive member is that it does not cause an industrial pollution.
In fact, using an organic photoconductive material as a charge generation substance can make it possible to freely select a wavelength range of a light sensible by an electrophotographic photosensitive member. For example, a series of azo pigments disclosed in Japanese Unexamined Patent Laid-Open No. 61-272754 as well as in Japanese Patent Laid-Open No. 56-167759 have exhibited a high sensitivity in the visible light region. Further, the substances disclosed in Japanese Patent Laid-Open No. 57-19576 as well as in Japanese Patent Laid-Open No. 61-228453 have shown a high sensibility even in the infrared region.
Among the charge generation substances disclosed in the aforementioned patent publications, those exhibiting a high sensitivity in the infrared region have been used in a laser beam printer (which will sometimes be referred to as LBP in the following description) and in a LED printer, each of which is adapted to digitally form electrostatic latent images. In fact, the needs for using such charge generation substances has become more and more frequent.
Recently, laser beam printer and LED printer have become a main trend printer in the commercial market. As a result, conventional printers having resolutions of 240, 300 dpi have become those having resolutions of 400, 600, 1200 dpi.
On the other hand, various copy machines have been improved in order to be equipped with more and more functions, and this has formed a rapid development in the direction of digitalization. In particular, digital machine involves the use of a method for forming electrostatic latent images using a laser beam, and this has become a main trend in this technical field. Similarly, recent printer also involves the use of a laser beam for forming electrostatic latent images, thereby obtaining an improvement in its resolution.
Usually, an electrophotographic photosensitive member for use in digitally forming electrostatic latent images is required to have the following properties.
(1) Capable of charging in dark position to a certain potential.
(2) Dispersing amount of charges in dark position is small.
(3) Capable of rapidly dispersing charges by virtue of a light irradiation.
In particular, with regard to the above (3), an electrophotographic photosensitive member is required to have a high sensitivity in the infrared region.
On the other hand, most phthalocyanine compounds have a high sensitivity in the infrared region and thus have been widely used as a charge generation substance contained in an electrophotographic photosensitive member. Particularly, in recent years, there has been widely used an oxytitanium phthalocyanine as a material having a high sensitivity in the infrared region. In addition, Japanese Patent Laid-Open No. 5-188615 has disclosed an electrophotographic photosensitive member formed by using a chlorogallium phthalocyanine, and Japanese Patent Laid-Open No. 5-249716 has disclosed an electrophotographic photosensitive member formed by using a hydroxygallium phthalocyanine.
In this way, each of the aforementioned conventional electrophotographic photosensitive members is formed by using a phthalocyanine compound as a charge generation substance and is found to have an extremely high sensitivity not only in the visible light region but also in the infrared region. This is because a phthalocyanine compound usually has a relatively high quantum efficiency and is capable of generating a great number of carriers. Although, at the present time it is still not quite clear as to why a phthalocyanine compound can generate a great number of carriers, a possible reason for this phenomenon is supposed to be the presence of oxygen and some other impurities.
However, in the case where a great number of carriers have been generated, if the electrons having the same amount as the holes (injected into the charge transport layer) can not quickly move towards a support portion, the electrons remaining in the charge generation layer will become excessive, forming a kind of memory easy to effect a potential change.
In principle, it is allowed to consider that the electrons remaining in the charge generation layer will usually proceed (due to a certain possible reason) to an interface between the charge generation layer and the charge transport layer, thus reducing a barrier efficiency of the holes injected in the vicinity of the interface.
In fact, when an electrophotographic photosensitive member is formed by using a phthalocyanine compound as a charge generation substance, the barrier efficiency will be reduced and this fact can be confirmed by finding a reduced bright position potential and a reduced residual potential during a continuous printing process. For example, in a development process (or a so-called reversal development process) where a dark position potential portion often used in a conventional printer serves as a non-developing portion and a bright position potential portion serves as a developing portion, positions which have received a light during a former printing process will offer a quick sensitivity. Accordingly, once an entire black image is removed during a next printing process, there will occur a so-called ghost phenomenon in which a formerly printed portion will be floated out.
The above phenomenon is particularly remarkable when an electrophotographic photosensitive member contains an intermediate layer serving as an adhesive layer for supporting the charge generation layer. In particular, under an environment of a low temperature and a low humidity, since a volume resistivity against the electrons in the charge generation layer as well as in the intermediate layer will be increased, the electrons will easily fill the charge generation layer, resulting in a problem that the aforesaid ghost phenomenon is more likely to occur.
Similarly, if a reversal developing electrophotographic process employs a laminated type of a electrophotographic photosensitive member whose charge generation layer contains a phthalocyanine compound, the aforesaid ghost phenomenon will also occur. In order to solve the above problem, a commonly used method requires that an electrophotographic photosensitive member""s first rotation involving a charging voltage drop is not used to form image (i.e., forming an idle rotation), but only the processes from a second rotation onward are used to form image, since each of these later processes has a stable charging voltage. Alternatively, a charge removal process is carried out by virtue of a light exposure conducted prior to a charging process, thereby avoiding the aforementioned problem. In fact, the above-described method is found to be useful in a reversal development type printer which is a conventional printer having a relatively low printing speed (for example, 10 or fewer A4 papers per minute). This is because an electric charger used in such a conventional printer usually has an extremely sufficient charge control capability, thereby effectively preventing the occurrence of the aforesaid ghost phenomenon. Moreover, since a certain time period is needed for data to be transmitted from a computer to a printer, the aforesaid idle rotation (the first rotation) will not cause any problem. However, with regard to recent digital copy machines which are required to perform a direct quick copying, the aforesaid idle rotation (the first rotation) will become an obstacle against a present trend of realizing a high speed copy machine.
Moreover, in the case where a light exposure prior to an electric charging is performed, in order to sufficiently attenuate an electric charging voltage, an exposure amount is required to be several times to 20 times an image exposure amount. As a result, an electrophotographic photosensitive member will get deteriorated due to the light exposure, and a dark position potential as well as a bright position potential during a continuous printing process will all change greatly. Accordingly, it has long been desired to research and develop an improved electrophotographic apparatus and an improved image formation method to ensure that all the rotations including a first rotation of the laminated type electrophotographic photosensitive member can be used to form image.
On the other hand, with regard to an electrophotographic method, when image is formed by carrying out several processes including electric charging, light exposure, development, transfer, fixing and cleaning, a conventional electric charging process is almost a process in which an electric charging occurs by virtue of corona generated by applying a high voltage (DC voltage: 5 to 8 kV) to metal wires.
However, the above-described electrophotographic method has been found to have the following problem. Namely, during corona generation, corona products such as ozone and NOx will change the surface structure of an electrophotographic photosensitive member, resulting in unclear image and image deterioration. Moreover, the metal wires are likely to be contaminated and the contaminated wires can bring about harmful influence to the quality of formed images, resulting in broken image and black stripe. In particular, an organic electrophotographic photosensitive member whose photosensitive layer is formed mainly by an organic photoconductive material, has been found to be chemically unstable as compared with an inorganic electrophotographic photosensitive member such as a selenium electrophotographic photosensitive member and an amorphous silicon electrophotographic photosensitive member. Moreover, the above organic electrophotographic photosensitive member is likely to get deteriorated since it is easy to receive a chemical reaction (mainly oxidation) once it is exposed to corona products. For this reason, in the case where the aforesaid organic electrophotographic photosensitive member is repeatedly used under a condition of corona charging, the deteriorated photosensitive member will produce unclear images and its sensitivity will be decreased. As a result, image concentration will become thin so that image itself will become faint, while the photosensitive member itself has only a reduced life time.
Furthermore, during the corona charging process, an electric current flowing towards the electrophotographic photosensitive member is only 5 to 30% of an entire current, with most amount of the current flowing to a shield plate, thus making the corona charging to have only a bad charging efficiency.
In order to solve the above problems, there has been suggested an improved charging method called contact type electric charging not involving the use of a corona discharger, as disclosed in Japanese Patent Laid-Open No. 57-178267, Japanese Patent Laid-Open No. 56-104351, Japanese Patent Laid-Open No. 58-40566, Japanese Patent Laid-Open No. 58-139156, Japanese Patent Laid-Open No. 58-150975.
In detail, a contact type electric charging process requires that a charging means such as an electrically conductive resilient roller (to which a DC voltage of about 1 to 2 kV has been applied) is caused to get in contact with the surface of an electrophotographic photosensitive member, thereby electrically charging the surface of the electrophotographic photosensitive member with an amount of electric charges and thus enabling the surface to have a predetermined potential.
Particularly, in recent years, the above-described contact type charging method has been incorporated into many electrophotograph apparatus, thus serving as a most commonly used electric charging method. In fact, a contact type electric charging method is almost always carried out by applying a voltage to an electrically conductive roller.
However, although the above-described contact type electric charging method has been proved to have an excellent charging efficiency, since an electric charging means such as an electrically conductive roller is caused to get direct contact with an electrophotographic photosensitive member, it has been known that a discharge insulation is likely to be broken due to even a very small defect or an ununiform portion existing within the electrophotographic photosensitive member.
On the other hand, in the case where an electrophotographic apparatus uses a semiconductor laser having a single wavelength (such semiconductor laser represents a main trend laser at present time), if the surface of a support portion for supporting an electrophotographic photosensitive member is a smooth surface, an interference phenomenon of the laser light will occur, causing an interference fringe on an image. In order to prevent such an interference fringe, it is necessary to make rough the surface of the support portion. However, such a rough surface will similarly damage a desired insulation, as would be caused by a defect existing within the electrophotographic photosensitive member. Accordingly, the roughness of the support portion, the thickness of films formed on the electrophotographic photosensitive member, the field intensity and the electric charging method are all required to be correct and appropriate.
In use, contact type electric charging can be classified into two different methods, with one applying only DC voltage, and the other overlapping an AC voltage on the DC voltage. In fact, the later method is more widely used than the former. This is because the use of the later method can avoid the damage of the discharge insulation of the electrophotographic photosensitive member (such an insulation damage would otherwise be caused due to an ununiformity of an electric charging as well as a direct application of DC voltage. In fact, an ununiformity of the electric charging will produce a stripe-like charged trace, having a length of 2 to 200 mm and a width of 0.5 mm or less, arranged in a direction orthogonal to the moving direction of an electrically charged surface. As a result, white stripe will occur during a normal development (white stripe occurs in solid black image or half tone image), while black stripe will occur during a reversal development, thus forming image defects.
In order to solve the above-discussed problem so as to improve the uniformity of an electric charging, there has been suggested a further method in which an AC voltage is overlapped on a DC voltage so as to apply tow kinds of voltages to an electric charging means (Japanese Patent Laid-Open No. 63-149668).
A method for overlapping an AC voltage on a DC voltage requires that an AC voltage (VAC) is overlapped on a DC voltage (VDC) so as to apply a pulsating current voltage, thereby ensuring a uniform electric charging. At this time, in order to maintain a uniformity of an electric charging so as to prevent image defects such as white spot in the normal development and to prevent black spot as well as overburden in the reversal development, it is necessary for the overlapping voltage to have an interpeak potential difference (VP-P) which is 2 times (or more) the DC voltage.
However, if an overlapping voltage is increased in order to prevent the aforesaid image defects, a maximum applied voltage of the pulsating current voltage will bring about a damage to the discharge insulation in even very small defect portions existing within the electrophotographic photosensitive member. In fact, such sort of insulation damage is particularly remarkable in the case where an electrophotographic photosensitive member is an organic electrophotographic photosensitive member (OPC) whose voltage withstanding capability is relatively low. At this time, the normal development will suffer from an image reverse occurring in the longitudinal direction of the contacting portions, while the reversal development will suffer from an image black girdle. Furthermore, in the case where pin holes have occurred, such pin holes will server as conductive path for electric current to leak therethrough, causing a problem that a voltage applied to an electric charging means will be decreased.
In addition, when an AC voltage is overlapped during an electric charging, since a discharge current during the electric charging is larger than the case where only DC voltage is applied, the consumption amount of the surface layer of the electrophotographic photosensitive member will become large, thus bringing about a problem that the electrophotographic photosensitive member has only a deteriorated durability. In the case where an electrophotographic photosensitive member is formed by laminating a charge transport layer on a charge generation layer, a sufficient life time of the electrophotographic photosensitive member can be ensured by increasing the thickness of the charge transport layer.
However, an increase in the thickness of the charge transport layer will cause a decrease in the repeatability of electrostatic latent image. If it is a digital apparatus, the repeatability of exposure spot at each dot will become deteriorated. As a result, it is difficult for a digital apparatus (for which research has long since been carried out in order to improve its fineness) to ensure both a good picture quality and a long life time.
Accordingly, it is an object of the present invention to provide an improved electrophotographic photosensitive member which employs a phthalocyanine such as oxytitanium phthalocyanine and hydroxygallium phthalocyanine as its charge generation substance, can maintain a high sensitivity (serving as an excellent electrophotographic property), and can provide images free from ghost phenomenon. It is another object of the present invention to provide an improved process cartridge and an improved electrophotographic apparatus, each containing the improved electrophotographic photosensitive member.
It is a further object of the present invention to provide another improved electrophotographic photosensitive member which is so formed that even when a DC contact charging method is employed for effecting an electric charging, it is still possible to prevent the occurrence of black stripe having a length of 2 to 200 mm and a width of 0.5 mm or less (in a direction orthogonal to the moving direction of a charged surface), as well as the occurrence of black spot, (both the black strip and the black spot are caused due to an ununiform electric charging), thereby making it possible to elongate the life time of the photosensitive member and provide image having a high and stable picture quality. It is a still further object of the present invention to provide an improved process cartridge and an improved electrophotographic apparatus, each containing the improved electrophotographic photosensitive member.
According to the present invention, there is provided an electrophotographic photosensitive member including a support portion, a charge generation layer formed on the support portion, a charge transport layer formed on the charge generation layer, characterized in that the charge generation layer contains a phthalocyanine compound, the charge transport layer has a thickness which is at least 9 xcexcm but not larger than 18 xcexcm, the electrophotographic photosensitive member in every 1 cm2 area has an electrostatic capacity (C) of 130 pF or more, a maximum height (RmaxD) of a surface roughness of the support portion, an average roughness (Rz) of 10 points, an arithmetic average roughness (Ra), and an irregularity average interval (Sm) have been set to satisfy the following conditions:
1.2 xcexcmxe2x89xa6RmaxDxe2x89xa65.0 xcexcm
1.2 xcexcmxe2x89xa6Rzxe2x89xa63.0 m
0.15 xcexcmxe2x89xa6Raxe2x89xa60.5 xcexcm
30 xcexcm less than Smxe2x89xa680 xcexcm
Further, according to the present invention there are provided a process cartridge and an electrophotographic apparatus each containing the above-described electrophotographic photosensitive member.